The printing industry now nearly exclusively utilizes photographic or digitally imaged printing plates to produce a printed image. Although this invention is primarily aimed at flexographic printing, it could be used for letterpress and offset printing, rotary screen printing as well as in any operation where it is desirable to wrap a flat planar item around a concave or convex (oval, a round, etc.) object or more often a cylinder or to form a flat planar item into a circular or tubular shape. In some instances, these “printing plates” are utilized only one time due to wear, storage concerns, economics, or obsolescence after first use, while in other instances, the printing plates will be re-used and are therefore saved.
Many printing presses utilize a circular drum (commonly called the “plate cylinder” or “print(ing) cylinder”) on which a printing plate containing typically an etched “raised” or “reverse” image to be printed is mounted. By rotating the plate cylinder and printing plate assembly first in contact with ink and next in contact with the substrate to be printed (paper, plastic, foil, etc.), the inked image is transferred from the plate to the paper. In the offset lithography process, a transfer drum called the “blanket” is between the printing plate and the printed substrate. Printing presses often have multiple printing units or “stations” such that the substrate, web or sheet, passes from one unit to the next. Other converting operations such as die cutting, slitting, laminating, or embossing may precede or follow the printing units, or be inserted between the printing units, or be in separate machines or multiple passes through the same machine. The web or sheets may be processed multiple times in the same or separate machines.
When printing a one color product, proper alignment of the printing plate on the plate cylinder is important to maintain proper orientation to the substrate and possibly to other converting operations. When printing in multiple colors, the printing press must normally have one plate cylinder and printing plate assembly for each color to be printed. In such cases, proper alignment (“registration”) of the printing plates on the plate cylinders is essential, otherwise the finished product will have improperly aligned images and colors in addition to improper alignment to the substrate and other converting operations.
In flexographic printing, the imaged printing plate is usually attached to an expensive plate cylinder using “flexographic mounting tape” or “stickyback”. Each plate cylinder for a 16-inch “narrow web” press typically costs several hundreds of dollars while plate cylinders for “wide web” presses cost substantially more. Flexographic mounting tape has either a solid core or foam core (usually rubber or polymer) with adhesive applied to both sides. If a plate is to be used in a subsequent run of the same or similar product, it may be desirable to keep it mounted on the plate cylinder.
Since keeping a printing plate mounted on a plate cylinder would tie up that expensive plate cylinder making it not available for other jobs, an alternative has been employed where the plate is mounted on an intermediate shell or sleeve that slides on and off the plate cylinder. These sleeves are usually metal or some plastic or composite material. While less costly than plate cylinders, sleeves are still quite expensive. Many printers have cited the benefits of sleeve mounting, but do not use it because a high investment in a large inventory of blank sleeves matched in diameter to the various diameters of plate cylinders in stock must be maintained to accommodate new jobs as they are scheduled. In addition, storage space required by mounted or blank sleeves is greater than for flat un-mounted printing plates.
The alignment accuracy of plate mounting is critical to produce high quality printing and to minimize waste due to mis-alignment or “mis-register” caused when trying to fit two or more images on top of each other in exact alignment, or when trying to achieve alignment with another operation such as die cutting or embossing. Mis-registration leads to increased manufacturing cost. Substrate and ink are usually large expenses to printing companies and they constantly try to minimize their waste. Lower waste also means shorter production time and thus reduced labor and overhead costs. Often only a specific limited quantity of substrate is available or allocated to produce the required quantity of printed product. Increased waste usually reduces the final quantity of product produced thus reducing the amount of finished product that may be billed for. Since customers usually have a tolerance on the quantity ordered, such as plus or minus 5%, reduction in production quantities could mean a missed sales opportunity. In some cases, if waste is too high, it may be required to purchase additional substrate and re-run a job to produce within the acceptable quantity range.
The major reasons for the need for accurate plate mounting (sleeve or the current system) include, but are not limited to: (1) some minor print mis-registration may be acceptable and in-specification, say +/−0.003 inch. However, out-of-specification production (waste) percentage increases with the amount of plate mis-alignment because of press registration float (relative movement or drift of the printed image locations in different printing units, either or both in the left-to-right or in the front-to-back directions), even on machines equipped with automatic tension, web guiding and automatic registration equipment. This is caused by: variation in substrate parameters; minor inaccuracies in machine drives, gears, cylinders, etc.; environmental conditions such as temperature and humidity; and other causes. Given this situation it is therefore highly desirable that printing plates be mounted as accurately as possible giving maximum registration latitude to other elements of the process.
A second reason for the need for accurate plate mounting arises if the press operator is dissatisfied with the plate mounting accuracy. Material waste and time accumulate as the operator first attempts to bring the job into acceptable running register. He or she may stop the setup or running processes and call for a re-mounting. Thus, the press, which may cost several hundred thousand dollars for narrow web to several million dollars for wide web and has a high hourly standby cost, is idle and not producing. Thirdly, print quality often suffers as more marginally out-of-register work is accepted and shipped. Finally, operators when challenged with plates marginally mis-aligned will need to focus more on the registration element of the process and subsequently run the machine slower and focus less on other elements of the process, resulting in lowered quality, slower running speeds and operator frustration. All these factors contribute to reduced productivity and increased cost.
A number of plate mounting systems are commercially available to mount plates directly on the cylinders or onto the sleeves. These systems generally utilize a method of alignment of two points on the surface of the plate, making the assumption that if all multiple images are in registration relative to two points, the entire composite multi-color (or multi-plate) image will be in registration. Usually registration marks are located such that a line drawn thru them should be perpendicular to the direction that the web or sheet moves thru the press. Most plate mounters are based on alignment of these marks parallel with the axis of the plate cylinder. The systems generally use CCD cameras or microscopes to facilitate this alignment, sometimes in conjunction with a hairline parallel to the plate cylinder axis. Some systems employ attachment points (holes or protrusions) that can be mechanically aligned. The same systems are used to mount plates onto plate cylinders or onto sleeves, so the use of sleeves this way in and of itself does not improve the registration accuracy.
These systems have served the industry well, but they have some limitations. First, not all cylinders are exactly of the same diameter. Also, sometimes cylinders have “taper” meaning that the diameter/circumference is not uniform along its length. Diametric differences also can be introduced either by variations in the mounting tape or its application to the plate cylinder. Also, there is no way to assure that the individual who mounts the plates uses uniform pressure or tension on the plate, which can introduce variability. Such inaccuracies are too small to be seen by an operator without the aid of magnification, yet may cause an out-of-specification condition or reduce registration latitude.
Therefore, even if one end or the central region of the plates is properly aligned, there is no guarantee that the forward or following portions are registered (aligned). One method for providing near perfect alignment of a generally flat planar object such as a printing plate onto a cylinder for mounting on to a printing machine cylinder is disclosed in applicant's U.S. Pat. No. 7,628,110. Although the system and method disclosed in this patent provides significant advantages over the prior art, in some instances two drawbacks with this system and method exist. First, all of the plate cylinders which are going to be utilized with this method must be taken out of service and modified to include a hollow interior region and a number of holes which extend to the exterior cylinder wall surface in order to allow for the use of compressed air to “float” the circular plate and sleeve assembly as it is slid onto the plate cylinder. Alternatively, new plate cylinders could be purchased incorporating these features which are more expensive than conventional cylinders. Cylinders so modified may not then be usable for the standard prior art mounting methodology. Secondly, sleeves which are preformed into a cylindrical form require a storage methodology that takes more space than flat printing plates. Moreover, this method is not familiar to many in the industry and the barrier to entry and acceptance by those in the industry could be difficult to overcome.
The rotary screen process utilizes a screen formed into a tube with the screen ink and squeegee located within the tube. Typically the screen material is a metal or fabric mesh, which is coated with a photosensitive resist such as “Screeny” made by Gallus. It is exposed, etched or ablated in the flat and then wrapped into a tube with a small portion of overlap at the leading and trailing ends. The alignment at this stage is critical so that the images can be printed in register. The overlap is then bonded, usually by some sort of adhesive.
Accordingly, what is needed is a new system and method for forming a printing plate mounted on a printing cylinder which is within the skill of those workers in the industry and somewhat familiar to them, and is both highly accurate and inexpensive and a system, method and analytical algorithms for measuring, recording, controlling and reporting the accuracy of printing plate mounting and alignment for a single plate, and for a set of multiple plates used together for a printing job and to indicate acceptability or suggest corrective actions.